In the HDCP (High-Bandwidth Digital Content Protection) protocol used in HDMI (High Definition Multimedia Interface) and DVI (Digital Video Interface), an encrypted frame signal referred to as Control 3 (CTL 3, or CTRL 3) is used to indicate that a frame of video is encrypted. The CTRL 3 signal appears during the vertical synchronization interval. CTRL 3 is transmitted by a source device to a sink device to signal that the associated frame is encrypted. When the sink device receives CTRL 3, it increments a counter that is used to establish a new decryption key for the received frame. A similar counter is incremented at the video source and the encryption key is similarly incremented. Periodically, the source checks to see that the sink device is synchronized. If not, an authentication process is carried out. This is the same authentication process used when communication between a source device and a sink device is first initiated.
If the CTRL 3 signal is not received at the sink, the source and sink counters will be mismatched, potentially resulting in unintelligible video decoding and unpredictable screen images (snow, black screen, etc). Every 128 frames (approximately every two seconds) the source verifies that the sink is correctly synchronized. When an unsynchronized situation is detected, the devices re-authenticate to establish synchronization.